Well, the most obvious answer is to provide the correct authentication/credentials.
The specifics on how to do that depend on several factors - first off, which mail application are you using? Apple Mail? Gmail? Outlook?
Then you need to check that applications' settings for accounts. In Apple Mail's case, it uses Settings -> Apps -> Mail -> Mail Accounts. Outlook and Gmail likely have other settings.
If checking/changing your password there doesn't work, you'd need to provide more details as to the account you're using (is this an iCloud email account? gmail? company account? since the next steps would be dependent on knowing that.